NUMBER TWENTY NINE - OCTOBER 1998

Many lakes have been damaged by human activities but it is unusual for there to be a full documentation of their history and for funds to be available for exhaustive scientific investigation before a restoration programme begins. The key to success in most cases lies in recognition of symptoms such that informed guesswork can produce a likely diagnosis. This is not unlike doctor-patient relationships! The lake at Llandrindod Wells in mid-Wales is a case in point.

It was formed late in the 19th century by damming a stream to create an amenity for the developing spa town. In the 1950s it is said to have had extensive plant beds and a brown trout population. The town decided to kill the 'weed' in the 1950s by adding a ton and a half of superphosphate to stimulate algal growth. This is unlikely to have made any difference as the lake is well flushed and the concentration created would have been modest.

However, at about the same time, the trout were removed and coarse fish, including common carp, were added. Since then the lake has been turbid and by the early 1990s, notices warning of potentially toxic blue-green algal blooms were posted at the lake. In the early 1990s, the local District Council asked the University of Liverpool to recommend what should be done to clear the water of blooms and re-establish it as an amenity with a water lily fringe. A brief scientific investigation was possible and this revealed a much higher total phosphorus concentration than could be explained by the inflows, which drained woodland and low intensity agricultural upland. Furthermore, the lake had a high carp population and a huge number of small bream. There were no fish predators. We concluded that the carp had acted as a forward switch and that the fish were mobilising a great deal of phosphorus from the sediments. The small bream were compounding the situation by their zooplanktivory.

It was possible to drain the lake and we recommended that the fish be removed after drainage. This was not welcome to the local carp angling club for the lake was well known as a carp fishery. It is not possible simultaneously to have a clear, amenity lake and a productive carp fishery and the District Council accepted this. The recommendation was then to refill the lake from the inflows, plant in starter populations of lilies and other plants, then restock with rudd, roach, pike and crucian carp. The common carp were to be sold on to angling clubs wishing to stock private waters. With some delays due to problems in draining the lake the plan was carried out and the lake is now recovering and is much clearer than it has been for decades.